Hypertensive heart disease is a relatively lethal manifestation of sustained systemic blood pressure elevation in the human. Following an increase in systolic loading, heart muscle hypertrophies to an extent that systolic tension per unit tissue mass remains close to normal levels. However the mechanical properties of hypertrophied heart muscle are different from normal. Hypertensive heart disease is the clinical manifestation of the quantitative and qualitative alterations in myocardium that facilitate initial compensation for increased pressure loading, but inevitably lead to the failure of the organ. The predictable age and sex-related levels of systemic hypertension in the spontaneously hypertensive rat and the availability of an in-bred control group derived from the same breed as the hypertensive animals, present a unique opportunity to quantitate the natural history of hypertensive cardiac disease. Force-velocity and active and resting muscle stress-strain curves will be obtained using left ventricular columnar carneae preparations and an isotonic afterloaded quick-release technique. Isometric data will also be obtained. An electromagnetic ergometer will be used to program the mechanical perturbations so as to relate rates of strain with force levels in the activated and resting preparation. The mechanical measurements will be used to document the response of hypertrophied heart muscle to a decrease in systemic pressure to normal levels. It is of great importance to know the extent to which the mechanical alterations associated with hypertrophy are reversible, and the relationship of the extent of reversibility to the age of the rat and duration of the pressure elevation.